This invention relates in general to a vehicular braking system, and in particular to a valve control scheme using non-driven wheel brake de-isolation for use in an anti-lock and traction control braking system.
Vehicles are commonly slowed and stopped with hydraulic braking systems. These systems vary in complexity but a base brake system typically includes a tandem master cylinder, pressure fluid conduit arranged in two similar but separate brake circuits, and a wheel brake cylinders in each circuit. The master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the pressure fluid conduit in both circuits to actuate brake cylinders at the wheels and slow the vehicle.
Base braking systems typically use a brake booster to provide a force to the master cylinder which assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated. A hydraulic booster uses pressurized fluid in a brake booster to move the master cylinder piston thereby increasing the master cylinder pressures generated when the driver applies the brakes. Hydraulic boosters are commonly located adjacent the master cylinder piston and use a boost valve to control the pressurized fluid applied to the booster. Typically the boost valve is connected with the booster in the master cylinder assembly and mechanically coupled to the brake pedal for proper operation.
Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive braking pressure thus causing one or more wheels to lock such that excessive slippage between the wheel and road surface takes place. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control.
Advances in braking technology have led to the introduction of anti-lock braking systems (ABS). An ABS monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. While such systems arc typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels.
Electronically controlled ABS valves, comprising isolation valves and dump valves, are located between the master cylinder and the wheel brake cylinders and perform the pressure regulation. Typically, when activated, these ABS valves operate in three pressure control modes; pressure apply, pressure dump and pressure hold. The isolation valves allow brake pressure into the wheel brake cylinders to increase pressure during the apply mode, and the dump valves release pressure from the wheel cylinders during the dump mode. Wheel brake pressure is held constant during the hold mode.
A further development in braking technology has led to the introduction of traction control (TC) systems. Additional valves have been added to existing ABS to provide a braking system which controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, braking pressures greater than the master cylinder pressure must quickly be available when the vehicle is accelerating.
During TC the electronically controlled ABS valves provide similar apply, bold and dump modes to regulate the fluid pressure at the driven wheel brake cylinder. The ABS valves isolate the wheel brake cylinders from the master cylinder during TC by closing off a direct path therebetween. Braking pressure at the wheel brake cylinders can be achieved which is greater than the pressure at the master cylinder. A pressure operated redundant brake switch is added to the braking system to provide information about the status of the hydraulic brake circuit. The switch is actuated whenever the pressure at the master cylinder exceeds a first predetermined pressure. When the brakes are applied during traction control pressurized fluid from the master cylinder must be able to reach the wheel brake cylinders for base braking. The redundant brake switch indicates when normal braking has begun by sensing the increased master cylinder pressure. At this time, the ABS valves are controlled so that the master cylinder pressure can reach the wheel brakes and normal braking is resumed. It is desirable to control the ABS valves so that the wheel brakes are not isolated from the master cylinder throughout traction control so that the redundant brake switch can be eliminated.
During the hold mode the fluid pressures created by the pump can build. A bypass valve is used to alleviate the excessive fluid pressures by opening a closed loop return path between the outlet and inlet of the pump. The bypass valve opens when the fluid pressure becomes excessive. The valve must be built to handle the high pressure and perform reliably. If the valve should fail to open the fluid pressures could reach undesirable levels. It would be desirable to provide a valve which, when actuated, would prevent the pump from creating the high fluid pressures rather than venting the high pressure fluid. By preventing the pump from creating the high fluid pressures the current drawn by the pump motor is much lower, the durability of the pump improves and pump noise is reduced.